Kutz M. Handbook of materials selection

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2 OXIDATION 269

environment (humid air or aqueous environments) degrades the ﬁlm by the for-

mation of a less stable hydrated oxide.

An important criterion to predict the formation of a protective oxide is the

Pilling–Bedworth ratio,

deﬁned as the ratio of the molecular volume of the

oxide to the atomic volume of the metal from which the oxide is formed. If this

ratio is less than one as in the case of magnesium (V

MgO

⫽ 0.81), the oxide

grows under internal stress leading to a discontinuous, porous ﬁlm possessing

low protective properties. To explain the good stability of magnesium at low

temperatures, it was suggested that initially an unstable structural modiﬁcation

of MgO grows oriented with the metal substrate, building a very thin compact

layer. Above a critical oxide thickness, however, the normal cubic lattice forms

and the arising stresses lead to cracking of the ﬁlm.

Generally, it is recognized that the initial oxidation of Mg proceeds by: (1)

oxygen chemiesorption, (2) formation of the oxide layer (nucleation and lateral

growth), and (3) oxide thickening

5–8

; in some cases a ‘‘precursor’’ chemisorption

stage before oxide formation was claimed. Initially oxygen is adsorbed on the

clean metal surface and forms a chemisorbed monolayer. A rearrangement of

valence electrons of the metal and the gas is then necessary for the formation

of chemical bonds. In the second stage additional layers of oxide build up form-

ing oxide islands; place exchange and surface diffusion are the important factors.

The island growth was reported to be fast and linearly dependent on the oxygen

exposure; the oxide thickening stage following coalescence of the oxide islands

is slow and diffusion controlled.

The ﬁlm was reported to grow by a ﬁeld-

assisted cation transport mechanism described by Cabrera–Mott kinetics.

A recent study

showed that the oxide ﬁlm naturally formed on pure Mg as

a result of exposure to air is thin (20–50 nm) and dense. It exhibits an amorphous

structure, which is probably the reason for the protective behavior. In low-

humidity atmospheres, no further increase in ﬁlm thickness is observed. X-ray

photoelectron spectroscopy (XPS) data indicated a mixture of elemental Mg,

MgO, and 50–60 wt. % Mg(OH)

Mg–Al alloys form similar ﬁlms,

12,13

which are largely enriched in Al, es-

pecially for alloys containing more than 4% Al. Figure 1 shows that there is a

terminal value of about 35% Al that can be incorporated in the oxide: All alloys

containing more than 4% Al produce an oxide containing 35% Al; however, if

the alloy contains less than 4% Al, the Al level in the oxide never attains 35%

and becomes a function of the alloyed Al. The presence of Al in the oxide ﬁlm

causes no chemical shift of the XPS signal for MgO, indicating that the oxide

is a mixture of MgO and Al

The initial air-formed ﬁlm on commercial Mg alloys is also dense and pro-

tective.

The thin ﬁlm can be visualized by transmission electron microscopy

(TEM) analysis on ultramicrotomed cross sections (see Fig. 2); electron diffrac-

tion reveals its amorphous structure. Since there is no need for accommodation

between the lattice of matrix and oxide, less internal stresses will develop. In

addition, amorphous ﬁlms are in general regarded to have better protective prop-

erties than the crystalline ones.

In extended atmospheric exposures of Mg and Mg alloys, the presence of

humidity and acid gases as CO

and SO

alters the composition of the oxide

270 CORROSION AND OXIDATION OF MAGNESIUM ALLOYS

Fig. 1 Al concentration in the oxide on homogenized AM alloys (in order to dissolve the

␤

phase) as a function of oxygen exposure.

Fig. 2 TEM cross section of an oxide ﬁlm formed naturally on AZ91 alloys exposed

to the atmosphere.

ﬁlm. Hydroxides, carbonates, sulﬁtes, or sulfates will form preferentially, thus

reducing the ﬁlm stability.

2.2 High Temperature

Most of the work dealing with oxidation of Mg at high temperatures is rather

old.

16–20

The rate of oxidation of Mg alloys increases with the temperature obey-

ing an Arrhenius equation. Magnesium hydroxide is not stable at high temper-

atures and begins to decompose above 350

⬚C.

2 OXIDATION 271

Fig. 3 Oxidation kinetics of AM50 ingots at 450⬚C in natural air (after Ref. 21).

At temperatures below 450⬚C in dry oxygen or at 380⬚C in moist oxygen, the

oxide ﬁlm formed on magnesium is protective for considerable lengths of time;

16,18

the weight law governing the oxidation is parabolic. However, the MgO

ﬁlm that forms above 450

⬚C is not protective,

16,17,19

and a linear oxidation law

is observed. During this degradation of the ﬁlm, a porous, white oxide grows

over the surface, producing a constant oxidation rate. The micromechanism in-

volved in this reaction is yet to be investigated. The activation energy for oxi-

dation between 475 and 575

⬚C was found to be about 211 kJ/mol;

this value

may represent the true rate of the chemical reaction since in the linear regime

oxygen has free access to the reaction surface. Above 600

⬚C oxidation is ex-

tremely fast leading to ignition.

A transition in the mechanism of oxidation is also observed in commercial

Mg alloys, as exempliﬁed in Fig. 3 for high-purity AM50.

Two different

regimes were observed in the oxidation curve of AM50 ingots at 450

⬚C: Ini-

tially a protective oxide is formed with a parabolic kinetics (k

⫽ 2 ⫻ 10

⫺

/cm

䡠 s) and after about 100 min a porous and less protective oxide forms

with a linear kinetics (k

⫽ 1.7 ⫻ 10

⫺

g/cm

䡠 s). Only thin MgO ﬁlms as

formed at the ﬁrst stages of oxidation are dense and act as a diffusion barrier

for both, oxygen and metal cations. Above a certain thickness tensile stresses

caused by the unfavorable Pilling–Bedworth ratio are assumed to be too high,

and partial cracking of the ﬁlm is likely to occur, permitting the access of oxygen

to the metal surface. Additionally, if cracks allow the metal to become exposed,

Mg might, in view of its appreciable vapor pressure, volatilize and react in the

vapor phase with the oxygen. The initial protective period gradually shortens

with the temperature since the critical thickness is more rapidly attained at higher

temperature.

The effect of alloying with various metals nobler than Mg on oxidation

was studied by Leontis and Rhines in the temperature range between 400 and

575

⬚C;

their results are represented in Fig. 4. Most alloying elements, e.g., Al

272 CORROSION AND OXIDATION OF MAGNESIUM ALLOYS

Fig. 4 Effect of alloying elements on the oxidation of Mg at ⬃475⬚C

(after Ref. 22, values from Ref. 17).

and Zn, increase the rate of oxidation; only Ce and La improve the resistance

of Mg to oxidation. The authors suggested that the increase in oxidation rate

results from a lowering of the melting temperature by the various additions. The

reactions were considered to take place at the metal/oxide interface. In contrast

to Ce and La, which are enriched in the surface layers, all other alloying ele-

ments get slightly depleted in the oxide.

Ca and Be are two of the few elements that are more reactive to oxygen than

Mg. They are known to have a remarkable effect on the oxidation resistance of

Mg,

in particular when added together with Al. The addition of as little as

0.001% Be will increase the ignition temperature of Mg by as much as 200

⬚C.

Thus, small additions of Be during melting help to control Mg loss and minimize

the use of ﬂuxes. A protective BeO ﬁlm forms on the surface, reducing drossing

and increasing metal yield and cleanliness. These beneﬁts carry over casting. Be

provides the further advantage of precipitating Fe and Mg impurities from the

melt.

Recent studies

performed on conventional AM and AZ alloys at 450⬚C

conﬁrmed the beneﬁcial effect of RE elements predicted by Leontis and Rhines

and showed that Ca strongly improve oxidation performance; rapid solidiﬁcation

(RS) was observed to further reduce the oxidation rate. The results shown in

Fig. 5 for AM50 alloys indicate that the modiﬁed alloys are very resistant to

oxidation. As little as 1% of Ca is enough to reduce the oxidation rate to ex-

tremely low values, even for longer oxidation times. Rare earths (added in the

form of mischmetal, Mm) are not as effective as Ca, and slightly higher contents

are necessary to achieve similar performance (see Fig. 5a).

The oxide formed on alloys with conventional composition is thick and po-

rous, whereas on Ca-containing alloys it is very thin so that metallic glance

essentially remains after testing. The ﬁlm consists of nanocrystalline MgO as

2 OXIDATION 273

Fig. 5 Oxidation kinetics of AM50 ingots at 450⬚C in natural air: (a) effect of Ca and Mm addi-

tions and (b) effect of rapid solidiﬁcation (after Ref. 21).

showed by X-ray defraction (XRD) and TEM cross sections (for AM50 the grain

size

⬍40 nm). In alloys containing Ca an enrichment of Ca in the MgO ﬁlm in

particular in its outermost part was detected by means of atomic emission spec-

troscopy (AES). Similar Ca enrichment was found in binary Mg–Ca alloys ox-

idized at 500

⬚C, being identiﬁed as CaO.

Therefore, the suppression of the

oxidation rate of Mg at high temperatures can be attributed to the formation of

CaO, which replaces MgO near the surface and acts as a protective layer; similar

explanation was given for the protection of the Mg melt by Ca additions.

The

role of RE in the ﬁlm formation is still under investigation. There is some

evidence that the effect of Ca becomes detrimental at lower temperatures. For

example, a recent study on AM50 alloys (same compositions as in Fig. 5) in-

dicated that the presence of 1% Ca increases the oxidation rate at 300

⬚C, while

both unalloyed and RE-containing AM50 alloys undergo almost no oxidation at

this temperature.

Precipitation of Al–Ca intermetallics might be responsible

for this deterioration. Measurements at even lower temperatures, within the limit

for application of Mg alloys, were not performed yet. Since at lower temperature

kinetics becomes very sluggish, extreme long-term measurements would be nec-

essary.

As shown in Fig. 5b, an additional beneﬁcial effect can be achieved by means

of RS; the improvement due to Ca and RE remains in the RS material. Until

now the inﬂuence of RS on the high-temperature oxidation deserved very little

attention. It seems that the advantages of RS, e.g., microstructural homogeneity,

contribute toward the formation of more stable ﬁlms as in the case of aqueous

corrosion.

In sulfur-containing atmospheres the oxide ﬁlm produced in the high-

temperature range was observed to be more protective.

Therefore, addition of

into the atmosphere retards the oxidation of Mg at high temperatures as

274 CORROSION AND OXIDATION OF MAGNESIUM ALLOYS

well as during casting. In gases containing H

SorSO

magnesium alloys are

among the most oxidation-resistant materials.

Addition of CO

also suppresses

oxidation of solid Mg.

In summary, the interaction of oxygen with Mg leads to the formation of a

thin, protective oxide layer. However, temperature as well as moisture accelerate

the transition to less protective ﬁlms. At elevated temperatures the MgO ﬁlm

rapidly attains a critical thickness and starts to crack, thus allowing further ox-

idation. Ca and RE improve the protective properties of the MgO ﬁlm and retard

the transition into a porous ﬁlm. Water, if present in the environment, rapidly

undermines the protective ﬁlm leading to a less stable, hydrated oxide and elec-

trochemical dissolution of the metal. This process of aqueous corrosion is the

subject of the next section.

3 CORROSION

In spite of the great advantages of Mg alloys, their application as engineering

material is still restricted by their high susceptibility to corrosion in aqueous

environments, especially in the presence of chlorides or when they contain noble

metals as impurities. In this section the poor corrosion behavior of Mg alloys

and the presently available methods to improve it are discussed. Over the past

30 years important improvements, which include modiﬁcations of the bulk alloy

and surface, were achieved and nowadays most limitations can be successfully

overcome by the correct use of Mg alloys. For further information other recent

reviews are recommended.

27,28

3.1 Electrochemical Properties

In aqueous environments magnesium dissolves electrochemically according to

reaction (2) with the formation of magnesium hydroxide and hydrogen, a mech-

anism that is highly insensitive to the oxygen concentration.

If sites for easy

hydrogen discharge are available, corrosion can proceed rapidly.

↑

Mg ⫹ 2H O → Mg(OH) ⫹ H (2)

222

The hydroxide ﬁlm has a hexagonal structure with alternating layers of Mg and

hydroxide ions, thus facilitating easy basal cleavage. Cracking and curling of

the ﬁlm was observed, but it is not clear whether this is caused by the properties

of the ﬁlm or by the evolution of hydrogen gas. The Pilling–Bedworth ratio is

1.77, indicating that the ﬁlm is under compression. Thus, a combination of

internal stresses and the easy basal cleavage may account at least partly for the

cracking and curling of the ﬁlm.

The standard reduction potential of Mg is ⫺2.4 V; at higher potentials Mg

corrodes rapidly. Despite the anodic polarization effect arising from the forma-

tion of Mg(OH)

the open-circuit potential in neutral aqueous solutions still falls

below

⫺1.5 V. Only at pH values higher than about 11 can a stable layer of

Mg(OH)

form and provide protection against further corrosion (see Fig. 6).

This protection is highly dependent on the environmental conditions, and Mg

turns out to be opposite in character to Al. Mg is resistant to alkalies and poorly

buffered environments where the surface pH may increase due to the initial

3 CORROSION 275

Fig. 6 Pourbaix diagram showing the equilibria for Mg–H

O system at 25⬚C.

Lines (a) and (b)

identify the hydrogen and oxygen evolution reaction, respectively.

Mg(OH)

formation but not to acids. In contrast Al is resistant to weak acids

but attacked by strong alkalies.

3.2 Types of Corrosion

Mg alloys undergo both general and localized corrosion. The very electronega-

tive potential of Mg makes these alloys very prone to galvanic corrosion. The

galvanic attack can be internal due to microstructural components with more

noble potential (e.g., impurities or second phases) or external if Mg is coupled

with other metals. While metals with low hydrogen overvoltage (e.g., Ni, Fe,

and Cu) constitute efﬁcient cathodes for Mg and cause severe galvanic corrosion,

metals that combine an active corrosion potential with a high hydrogen over-

voltage (e.g., Al and Zn) are much less damaging. Internal attack can be mini-

mized by selecting high-purity alloys. To avoid external galvanic corrosion, one

of the following measures is recommended:

(1) Selection of galvanic compat-

ible dissimilar metals (e.g., Al alloys 5052, 5056, and 6061; or tin, cadmium,

or zinc-plated ferrous alloys). (2) Protection of Mg and dissimilar metal by

suitable surface treatments; painting of the dissimilar metal or even better of

both metals is recommended to avoid area effects. (3) Use of insulating washer

or gasket between Mg and dissimilar metals to prevent the completion of an

electrical circuit. (4) Inhibition of the galvanic cell by using chromates in the

sealing compounds or primers. (5) Proper design to avoid electrolyte entrapment.

Intergranular and crevice corrosion are not likely to occur in Mg alloys be-

cause the phases at the grain boundaries are always cathodic to the grain interior

and the corrosion reaction is insensitive to oxygen concentration differences.

276 CORROSION AND OXIDATION OF MAGNESIUM ALLOYS

Usually, corrosion concentrates at regions adjacent to second-phase particles and

boundaries. Some grains or particles might be undermined and fall out, resulting

in higher mass losses than solely due to electrochemical dissolution; this partly

explains the so-called negative difference effect observed during anodic polari-

zation of Mg.

As a naturally passive metal, Mg may suffer pitting corrosion. AZ91 in

chloride-containing environments exhibits a variety of localized forms of cor-

rosion, including pitting at the outset (initiation sites are few and associated with

intermetallic particles), ﬁliform corrosion at early stages of propagation, and a

cellular type of attack in the terminal stage.

33–35

The occurrence of ﬁliform cor-

rosion on uncoated surfaces of AZ91, which is not the case for unalloyed, bare

Mg,

suggests the presence of a more resistant air-formed oxide on the alloy.

Pure Mg is resistant to stress–corrosion cracking (SCC). However, alloying

with Al and Zn promotes SCC, which increases with increasing Al content.

Thus, Mg–Al–Zn alloys have the greatest susceptibility to SCC. Alloys con-

taining Zr are essentially free from the phenomenon. SCC in Mg alloys

15,37,38

usually transgranular occurring along twin boundaries or various crystallographic

planes. There is general agreement that hydrogen embrittlement is the dominant

mechanism. Since hydrogen cannot penetrate the passive ﬁlm unless it is already

damaged, pitting is likely to be the ﬁrst step on SCC. Therefore, solutions that

are nonactive to Mg (such as diluted alkalies, concentrated HF, and chromic

acid) or in which general corrosion predominates do not induce SCC. High-

humidity content, dissolved oxygen, and seawater accelerate SCC; heavy metal

content does not seem to inﬂuence it. SCC was studied mainly in Mg–Al systems

and information is still lacking for other Mg alloys, in particular for new emerg-

ing alloys, e.g., glassy alloys or those containing RE.

3.3 Environment and Surface Film

Owing to the very active electrochemical character of Mg, the corrosion behavior

of its alloys reﬂects the protective qualities of the surface ﬁlm in various envi-

ronments. As discussed in Section 2.1, in natural environments with low hu-

midity a stable MgO ﬁlm [or a mixture of MgO and Mg(OH)

] forms on

magnesium and protection is essentially perfect. Atmospheric corrosion is a

threat only in the presence of humidity that leads to condensed water. The cor-

rosion rate of magnesium during exposure to humid air is less than 10

⫺

mm/

yr in the absence of condensation, while it becomes greater than 1.5

⫻ 10

⫺

mm/yr when water condenses on the surface.

Hence, for indoor service Mg

alloys serve well without any further protection.

Although stable and dense, the air-formed ﬁlm is permeable to water and

soluble Mg species. Exposure to humid air rapidly causes hydration of the oxide

and formation of thicker ﬁlms (e.g., 100–150 nm after 4 days) with a duplex

structure.

11,12

The outer layer of the ﬁlm (20–40 nm) is amorphous and similar

to the ﬁlm formed in dry air (see Fig. 2), but an additional hydrated layer is

formed adjacent to the metal. The hydrated layer reduces the passivity of the

metal surface by allowing Mg dissolution. This layer is quite sensitive to electron

irradiation and develops a cellular structure in the microscope; continued ex-

posure to the electron beam causes release of water and formation of nanocrys-

talline fcc MgO. Immersion in water leads to the formation at the outermost

3 CORROSION 277

Fig. 7 Schema of the three layered oxide formed on Mg and Mg–Al alloys in water-containing

environments. (Reprinted from Corrosion Science, 39, J. H. Nordlien, S. Ono, N. Masuko, and

K. Nisancioglu, ‘‘A TEM Investigation of Naturally Formed Oxide Films on Pure Magnesium,’’

surface of a third layer with plateletlike morphology, which probably grows by

precipitation of Mg

⫹

or other soluble Mg species that migrate outward from

the innermost layer. Figure 7 shows schematically the morphology of the layered

ﬁlm formed in the presence of water.

Films formed in water on Mg alloys are sensitive to temperature, agitation,

and contaminants.

The corrosion rate in water may increase by one or two

orders of magnitude when the temperature rises from room temperature to

around 100

⬚C.

Immersion in a small volume of stagnant water allows rapid formation of the

protective hydroxide ﬁlm. Owing to the initial dissolution of hydroxide, the pH

of the system rises until the point is reached where further dissolution is inhibited

and the metal undergoes passivation. Agitation can prevent the pH to rise so

that the solubility limit of Mg(OH)

is never reached and corrosion continues at

the initial high rate. For example, AZ31B shows little attack in stagnant distilled

water, but in constantly replenished water the corrosion rate is signiﬁcant (0.18

mm/yr).

The presence of small amounts of dissolved salts, in particular chlorides and

heavy metal salts, breaks down locally the protective ﬁlm formed in aqueous

environments. All solutions containing salts of heavy metals such as Fe, Ni, and

Cu are extremely aggressive because the heavy metal and/or heavy metal basic

salt can precipitate to form active cathodes on the anodic Mg surface.

Anions such as chloride, sulfate, and nitrate destroy the passivity of magne-

sium and cause severe damage. Saline environments such as marine atmospheres,

salty road splash, etc. are always harsh environments for Mg alloys, even when

protected. For this reason, the majority of corrosion data refer to salt spray test

278 CORROSION AND OXIDATION OF MAGNESIUM ALLOYS

and immersion in NaCl solutions operated according to the ASTM Standards

B117-90 and G31-72, respectively.

These data must be interpreted with caution

because salt based accelerate tests usually lead to higher corrosion rates than

marine atmosphere exposure and show very poor correlation with rural, urban

or industrial atmosphere exposure.

The CO

in the atmosphere dissolves in water to form carbonic acid, which

reacts with the Mg(OH)

ﬁlm to form carbonates.

While these ﬁlms are quite

protective, they are slightly soluble in water and do not provide complete pro-

tection over extended periods of time. The same is true for other acid-producing

compounds, such as SO

in industrial atmospheres. Exposure to wet SO

was

suggested to accelerate the corrosion of magnesium

through the conversion of

the more protective hydroxide and carbonate compounds to the highly soluble

sulfate and sulﬁte, which are then eroded. If Al is present in the alloy, the effect

of CO

is smoother due to the formation of hydrotalcite, MgCO

䡠 5Mg(OH)

䡠

2Al(OH)

䡠 4H

O, acting as a sealer for the hydroxide ﬁlm. In the presence of

chlorides, the accelerating effect of CO

on the atmospheric corrosion rate is

rather weak; it becomes more important only for a low degree of salt contami-

nation.

In the presence of ﬂuoride, the ﬁlm changes from magnesium hydroxide to a

very protective magnesium ﬂuoride and protection is greatly increased, even at

high temperatures. Mg is even used for constructing handling equipment for

concentrated HF. Chromate also forms highly protective ﬁlms on Mg, and chro-

mic acid is the basis for many protective surface treatments.

A good rule of thumb is that environments that are basic, neutral, or contain

ﬂuorine cause little or no corrosion, while environments that are acid do cause

corrosion of Mg and its alloys.

3.4 Improving Corrosion Resistance

Apart from the inﬂuence of the environment, the corrosion behavior of Mg alloys

depends (similar to other metallic materials) on the presence of impurities, com-

position (e.g., type and amount of alloying elements) and processing methods

(e.g., type of casting and heat treatment). However, the relative importance of

these factors is greatly ampliﬁed for all Mg alloys because of the very negative

corrosion potential of the matrix combined with the unstable nature of the sur-

face ﬁlm.

There are four main approaches that can be used to improve the corrosion

behavior of Mg alloys: (1) The ﬁrst approach is the production of high-purity

alloys with very low impurity levels; high-purity alloys appeared in the 1980s

and presently their use is standard practice. (2) Further important improvements

on corrosion can be achieved by alloying with beneﬁcial elements. This is a

rather old method, however, in recent years new compositions are been studied.

(3) Selective processing that optimizes the microstructure also enhances corro-

sion resistance; new processing techniques such as RS were shown to be very

promising. (4) A further approach, which is complementary to those inﬂuencing

the alloy itself, is the application of protective ﬁlms or coatings.

Control of Impurities

In general, the factor with the far strongest inﬂuence on the corrosion of Mg

alloys is the amount of cathodic impurities, particularly of those with low-